Welcome to TeX.SX! Could you add an example of text that shows the problem?
– egregFeb 9 '14 at 20:54

1

I think that the font has only regular and bold weights, no italic or slanted shape. Try amiri instead.
– ThérèseFeb 10 '14 at 1:31

Thank you for your answers. I'll post an example.. I tried replacing Traditional arabic by amiri, and have an error message..couldn't find the font amiri.. How can I install it ?.. But after.. the editor wants this font (Traditional arabic)...bon... Thank you
– Faouzi BellalounaFeb 10 '14 at 8:24

As the name suggests, it takes the regular upright letters and slant them automatically (using a simple transformation, I think).

The other method is to use a different font which does have a slanted version. This way Traditional Arabic will be used for upright letters, and whenever you use \emph or \textitfontspec will choose the other font. Notice that slanted version of Arabic fonts usually have Italic or Slanted after the font name. To use this method, define the font family thus:

I think the first method is probably preferable, since finding another font which is similar enough typographically might be really tricky. I use this first method for my thesis: my main Hebrew font has no slanted version (grosso modo, Hebrew has no italics, but some attempts at designing ‘italic-like’ letter forms for specific typefaces has been made), so I use AutoFakeSlant in order to obtain slanted version for citing titles of other works (cf. ‘The Hobbit’ in English). It works fine. Notice that for both Arabic and Hebrew one should use negative factor (I find -0.15 a reasonable choice), as they are written RTL, while LTR scripts should use a positive one.

In general, I advise against using bold and slanted in Arabic for emphasis: they work poorly in Arabic, especially the slant which is not always as visible and is quite alien to Arabic typesetting traditions. (Even though my own Amiri font has them, they are there to avoid the fake bold and slanted synthesized by GUI applications which are very very poor and outright break the font in many cases.) Traditionally, emphasis was done using a different calligraphic style, for example using an Nastaliq for emphasis with Naskh for the main text, or using a different font that has a bolder look, or using overlines, or different colors etc. So I suggest you employ one of those techniques based on the nature of the text your are setting.